Method and device for carrying out a reaction in liquid medium with gas evolution

ABSTRACT

The invention concerns a device for carrying out a reaction in liquid medium during which there is gas evolution. The inventive device comprises at least: a static mixer ( 3 ) wherein emerges at least a pipe ( 1 ) supplying liquid medium; a cyclone reactor ( 4 ) connected to said static mixer ( 3 ) and provided with a vent ( 5 ) for evacuating the gas or gases formed and means for evacuating ( 6, 7 ) the liquid medium. The invention also concerns a method whereby a reaction is carried out in liquid medium during which there is gas evolution. The invention can be used particularly for a reaction involving a peroxide such as hydrogen peroxide and, in particular, for reducing chlorine contained in a liquid effluent.

[0001] The invention relates to a process and a device for carrying outa reaction in liquid medium during which evolution of gas occurs.

[0002] The invention can be applied in particular to a reactionemploying a peroxide such as hydrogen peroxide and, in particular, tothe reduction of chlorine present in an aqueous effluent.

[0003] It is known to reduce chlorine having a degree of oxidation ofgreater than or equal to zero using hydrogen peroxide.

[0004] As examples mention may be made of the following known reactions:

NaClO+H₂O₂→NaCl+O₂+H₂O

CeOH+H₂O₂→HCe+O₂+H₂O

Cl₂+H₂O₂→O₂+2HCl

[0005] Also known, from European patent application no. 863 218, is amethod of recovering germanium from gaseous effluents originating fromchemical deposits in vapor phase which comprises a step in which, in amixer, using a peroxide such as hydrogen peroxide, hypochlorite ions(ClO⁻) are reduced to chloride ions (Cl⁻).

[0006] Another process, described in U.S. Pat. No. 5,354,435, proposesproducing chlorine dioxide from a solution of chloric acid (HClO₃). Thisprocess includes a step during which, in a conical reactor, a solutionof chloric acid is reacted with a reducing agent such as hydrogenperoxide.

[0007] As shown by the above reaction there is an evolution of oxygen,which generally gives rise to substantial foaming. Since this foaming isdisruptive, it is generally avoided by introducing a chemical antifoaminto the reaction medium.

[0008] Another solution consists in breaking the foam formed by means ofspecific equipment provided in the reactors, for example, by surfacespraying, or by means of a stirrer rotating at the surface in thefrothing mass.

[0009] The aim of the invention is therefore to provide simple, economicand easy-to-use equipment making it possible on the one hand toimplement the reaction under conditions which ensure a high degree ofsafety and on the other hand to obtain high kinetics, a high yield, andhigh productivity.

[0010] The invention accordingly provides a device for carrying out areaction in a liquid medium during which evolution of gas occurs, saiddevice being characterized in that it comprises at least:

[0011] one static mixer in which emerges at least one pipe for feedingwith liquid medium;

[0012] one cyclonic reactor connected to said static mixer and equippedwith a chimney for the discharge of the gas or gases formed and withmeans for discharge of the liquid medium.

[0013] The invention likewise provides a process in which a reaction iscarried out in a liquid medium during which evolution of gas occurs,this process being characterized in that it comprises the followingstages:

[0014] the liquid medium is introduced into a static mixer and thereaction is allowed to begin;

[0015] the reaction medium is transferred from the static mixer to acyclonic reactor;

[0016] the reaction is allowed to continue in the cyclonic reactor; and

[0017] the liquid medium exiting at the bottom from the cyclonic reactoris recovered, optionally after its entry into a gas/liquid separator.

[0018] Other features and advantages of the invention will emerge onreading the specification which follows and which is given by referenceto the single FIGURE attached, which represents diagrammatically thedevice according to the invention in its preferred embodiment.

DETAILED EXPOSITION OF THE INVENTION Device According to the Invention

[0019] The single FIGURE attached represents the device according to theinvention, in its preferred form.

[0020] This device is based on the combination of at least twoapparatus:

[0021] a static mixer 3 in which emerges at least one pipe 1 for feedingwith liquid medium;

[0022] a cyclonic reactor 4 connected to said static mixer 3 andequipped with a chimney 5 for the discharge of the gas or gases formedand with means for discharge 6, 7 of the liquid medium.

[0023] The static mixer 3 is of conventional type; it may, for example,consist of helicoidal elements.

[0024] The cyclonic reactor 4 is likewise of conventional type. Overallit has a conical or frustoconical form whose apex constitutes the bottompart. The bottom part may further be equipped with an antivortex device(not shown).

[0025] The cyclonic reactor 4 is fed with liquid medium at its top partvia a pipe 18 which connects it to the outlet of the static mixer 3.

[0026] Preferably this pipe 18 connecting the outlet of the mixer 3 tothe cyclonic reactor 4 is arranged such that the reaction medium enterstangentially into the cyclonic reactor 4. This makes it possible, ineffect, for better evolution of the gases present in the liquid medium.

[0027] The means for discharge 6, 7 of the cyclonic reactor 4 serve toevacuate the liquid medium from this reactor. They may therefore consistof a gutter 7 located at the level of the top part of the reactor.Discharge of the reactor is in this case effected by overflow. Thus,during operation of the reactor, the liquid medium overflowing at thetop of the cyclonic reactor 4 is collected by the gutter 7.

[0028] Alternatively, discharge may be effected by means of a pipeline 6situated at the bottom part of the cyclonic reactor 4.

[0029] According to one preferred embodiment of the device according tothe invention the static mixer 3 is fed not only by the first conduit 1but also by at least one second feed conduit 2.

[0030] According to another preferred embodiment of the device accordingto the invention a portion of the liquid medium collected by the gutter7 and/or the pipeline 6 is reinjected into one or other of the feedpipes, 1 or 2, generally by passage through a pump 8, the entirety ofthe device thereby forming a loop or recirculation system.

[0031] The remainder of the liquid is then removed from the loop bymeans of an exit pipe 9.

[0032] The determination of the amount of liquid medium to be reinjectedinto the feed pipe 1 or 2 can be calculated from the concentrationsmeasured by an assay probe (not shown) and supplied by an assay pipe 13situated preferably between the pump 8 and one of the feed pipes 1 and2. Of course, the amount of liquid medium withdrawn by the assay pipe 13is negligible.

[0033] The flow rates through the various pipes 1, 2, 9 and 13 areregulated by means of flow valves numbered 14, 15, 16 and 17respectively.

[0034] The realization of a loop brings the additional advantage thatthe temperature of the reaction medium can be adjusted to the optimumconditions of reaction kinetics and of gas evolution. This may involve,for example, cooling in the case of exothermic reaction. This loop mayalso make it possible to ensure a minimal speed within the static mixer.

[0035] In order to improve further the removal of the gas or gasespresent in the liquid medium it is desirable to provide a gas/liquidseparator 10, which can be a coalescer, and which is connected to themeans for discharge 6 and/or 7 of the cyclonic reactor 4. Where a pump 8is present in the device the outlet orifice of the separator 10 suppliesthe upstream side of the pump 8 via the pipe 11.

[0036] Accordingly, as can be seen in the FIGURE, the liquid medium fromthe gutter 7 and/or from the pipeline 6 undergoes additional separationin the coalescer 10.

[0037] The gas or gases emerging from the coalescer 10 may then, via apipe 12, rejoin the chimney 5, and the liquid medium, freed from thegases, exits from the coalescer 10 via the conduit 11, which takes itfrom the upstream side of the pump 8.

[0038] According to one advantageous embodiment of the invention means(not shown) are provided for introducing air or an inert gas into thecyclonic reactor 4. This makes it possible to prevent the accumulationwithin the cyclonic reactor 4 of the gas or gases produced by thereaction, which in certain cases could give rise to an inflammable orexplosive medium.

[0039] Means may also be provided for heating or cooling the mixer 3and/or the cyclonic reactor 4.

[0040] The device according to the invention may be utilized forcarrying out any reaction in a liquid medium in the course of which agas is formed which is liable to give rise to the formation of foam, andparticularly a reaction employing a peroxide. The operation of thedevice may be continuous or discontinuous.

[0041] The device according to the invention may be advantageouslyutilized as a dechlorination plant, especially for reducing the chlorinepresent in aqueous effluents.

[0042] Process According to the Invention

[0043] The process according to the invention may be implemented withthe aid of the device according to the invention that has just beendescribed.

[0044] Picking up the numbering of the device according to the inventionas used above, the process according to the invention thus comprises thefollowing stages:

[0045] the liquid medium is introduced into the static mixer 3 and thereaction is initiated in said static mixer 3;

[0046] the reaction medium is transferred from the static mixer 3 to acyclonic reactor 4;

[0047] the reaction is allowed to continue in the cyclonic reactor 4;and

[0048] the liquid medium exiting at the bottom from the cyclonic reactor4 is recovered, optionally after its entry into a gas/liquid separator10.

[0049] Generally speaking, the reaction starts in the static mixer 3 andends in the cyclonic reactor, thereby making it possible for the gas orgases formed to be removed in a very short time.

[0050] The reaction generally continues in the cyclonic reactor on thesurface to start with and then in the bottom volume, which can beadjusted by level regulation.

[0051] In the static mixer 3 a degree of progression of the reaction offrom 80 to 95% is generally achieved, thereby allowing high speeds to beattained within the cyclonic reactor 4.

[0052] When the liquid medium is extracted via the pipeline 6, thebottom of the cyclonic reactor 4 acts as finisher.

[0053] According to one preferred embodiment, air or an inert gas isadditionally introduced into the cyclonic reactor 4, for the reasons setout above.

[0054] It is likewise possible to heat or cool the mixer 3 and/or thecyclonic reactor 4.

[0055] The process according to the invention is applied to any reactionin liquid medium in the course of which a gas is formed which is likelyto give rise to the formation of foam.

[0056] The gas may be a product of the reaction (decomposition) of thereactant present in the liquid medium.

[0057] The liquid medium may be an aqueous medium or a solvent,depending on the type of reaction envisaged. The medium is preferablyaqueous.

[0058] The process according to the invention may be applied to anyreaction in a liquid medium comprising at least two reactants A and B,in the course of which a gas is formed which is capable of bringingabout the formation of foam. In this case it is preferred to use adevice according to the invention comprising a first pipe 1 for feedingwith first reactant A and a second pipe 2 for feeding with secondreactant B.

[0059] The reactant A can be a reducing agent or an oxidizing agent.

[0060] If the reactant A is a reducing agent it can serve, for example,to reduce a reactant B comprising a halogen having a degree of oxidationof greater than or equal to zero.

[0061] The reactant B then may therefore comprise at least one compoundchosen from the group consisting of chlorinated compounds, brominatedcompounds and permanganates such as sodium permanganate or potassiumpermanganate.

[0062] As chlorinated compounds mention may be made of chlorine,chlorine dioxide, hypochlorites such as sodium hypochlorite or calciumhypochlorite, hypochlorous acid and solid forms of chlorine.

[0063] As brominated compounds mention may be made of bromine,hypobromites, such as sodium hypobromite or calcium hypobromite, andhypobromous acid.

[0064] Where the reactant A is an oxidizing agent it may then oxidize areactant B comprising at least one compound such as sodium chlorite,potassium chlorite, calcium chlorite, cyanide compounds, sulfurcompounds and ferrous iron.

[0065] As reactant A it is possible for example to employ a reactantcomprising at least one peroxide such as hydrogen peroxide and alkalimetal peroxides such as sodium or potassium peroxide. Preference isgiven to using hydrogen peroxide.

[0066] Accordingly the process according to the invention may beadvantageously employed to reduce the chlorine present in aqueouseffluents.

[0067] The aqueous effluents, reactant B, generally comprisehypochlorite ions and/or chlorine and it is then preferred to select asreactant A an aqueous solution of hydrogen peroxide. The gas evolved isoxygen.

[0068] In particular the process can be used to treat an aqueouseffluent comprising:

[0069] from 1 mg/l to 10 g/l and preferably from 10 mg/l to 4 g/l ofCl₂; and

[0070] from 1 mg/l to 250 g/l and preferably from 10 mg/l to 130 g/l ofNaClO.

[0071] The reactant A and/or the reactant B may further comprise aconventional antifoam.

[0072] The process according to the invention is preferably carried outat temperature and under the following conditions:

[0073] temperature: from 0 to 110° C., preferably from 20 to 80° C.;

[0074] pressure: 0.5 to 3 bar, preferably from 0.9 to 1.3 bar;

[0075] pH: from 1 to 14, preferably from 5 to 12;

[0076] residence time in the static mixer: 0.001 to 100 seconds,preferably from 0.02 to 10 seconds;

[0077] residence time in the cyclonic reactor: 10 to 400 seconds,preferably 20 to 100 seconds; and

[0078] the gas/liquid separator 10 is a coalescer, the operatingvelocity of which is from 0.01 to 1 m/s, preferably from 0.05 to 0.8m/s.

[0079] It may be carried out continuously or discontinuously(batchwise).

EXAMPLE

[0080] The example which follows illustrates the present inventionthough without limiting its scope.

[0081] An aqueous solution A of hydrogen peroxide (H₂O₂) is preparedwith a concentration of 35% by weight.

[0082] Then an aqueous solution B is prepared which has the followingcomposition: NaCeO: 79.07 g/l NaCl: 62.10 g/l Na₂CO₃:   20 g/l H₂O:remainder to 11

[0083] An apparatus is then acquired like that shown in the attachedFIGURE.

[0084] The reactor has a diameter of 0.4 m in its top part and 0.3 m inits bottom part, and a height of 0.6 m.

[0085] The coalescer has a diameter of 0.1 m over a length of 1 m.

[0086] The static mixer has a diameter of 25 mm and a length of 1 m.

[0087] The feed pipe 2 provides a feed of solution A with a regulatingvalve and flow meter; the feed pipe 1 provides a feed of solution B withregulating value and flow meter.

[0088] The pipe 9 provides for removal of the treated solution so as toensure a constant level in the cyclonic reactor, with a regulatingvalve; the pipe 13 provides the feed of an assay probe for hydrogenperoxide in the solution treated.

[0089] The flow rate in the circuit is fixed between 1 and 5 m³/h.

[0090] The solution B is introduced at a rate of 200 l/h into thecircuit, at the inlet of the static mixer 3.

[0091] The solution A is supplied at a rate of 24 l/h, which iscontrolled from the results provided by the H₂O₂ assay probe suppliedvia the pipe 13.

[0092] The mixing of solutions of A and B allows oxygen to be formed atthe outcome of the following reaction:

NaClO+H₂O₂→NaCl+O₂+H₂O

[0093] In the static mixer 3 approximately 90% of the reaction iseffected, thereby allowing high rates to be achieved with a residencetime of less than 1 second. The outlet of the static mixer 3 to thecyclonic reactor 4 is located tangentially in order to allow maximumremoval of oxygen.

[0094] Within the cyclonic reactor 4 a level is maintained so as toallow a residence time of more than 20 seconds. This residence timeallows the reaction to be completed and the removal of the oxygen to beincreased. Located at the bottom of the cyclonic reactor 4 is acoalescer 10, with a transit rate of less than 1 m/s, in order to ensurethe coalescence and removal of oxygen mircobubbles. The liquid outlet 11of the coalescer 10 provides a supply to the pump 8.

[0095] Thus, with a fixed flow rate of solution B to be treated (200l/h), the adjustment of the flow rate of solution A around 24 l/h madeit possible to regulate the excess of H₂O₂ assayed by the probe ofbetween 30 and 100 mg/l.

1. A device for carrying out a reaction in a liquid medium during whichevolution of gas occurs, characterized in that it comprises at least:one static mixer (3) in which emerges at least one pipe (1) for feedingwith liquid medium; one cyclonic reactor (4) connected to said staticmixer (3) and equipped with a chimney (5) for the discharge of the gasor gases formed and with means for discharge (6, 7) of the liquidmedium.
 2. The device as claimed in claim 1, characterized in that saidmeans for discharge of the liquid medium comprise a gutter (7) capableof collecting the liquid medium overflowing from the top part of thecyclonic reactor (4).
 3. The device as claimed in claim 1, characterizedin that said means for discharge of the liquid medium comprise apipeline (6) situated at the bottom part of the cyclonic reactor (4). 4.The device as claimed in one of claims 1 to 3, characterized in thatsaid discharge means (6, 7) are connected to the feed pipe (1) andoptionally to an exit pipe (9).
 5. The device as claimed in one ofclaims 1 to 4, characterized in that a gas-liquid separator (10) isprovided in order to receive the liquid medium originating from saiddischarge means (6, 7), the outlet (11) for the liquids from thegas-liquid separator (10) being, if appropriate, connected to the feedpipe (1).
 6. The device as claimed in one of claims 1 to 5,characterized in that a pipe (18) connects the outlet of the mixer (3)to the cyclonic reactor (4) such that the reaction medium enters thecyclonic reactor (4) tangentially.
 7. The device as claimed in one ofclaims 1 to 6, characterized in that it additionally comprises means forintroducing air or an inert gas into the cyclonic reactor.
 8. The deviceas claimed in one of claims 1 to 7, characterized in that itadditionally comprises a second feed pipe (2).
 9. A process in which areaction is carried out in a liquid medium during which evolution of gasoccurs, characterized in that it comprises the following stages: theliquid medium is introduced into a static mixer (3) and the reaction isallowed to begin; the reaction medium is transferred from the staticmixer (3) to a cyclonic reactor (4); the reaction is allowed to continuein the cyclonic reactor (4); and the liquid medium exiting at the bottomfrom the cyclonic reactor (4) is recovered, optionally after its entryinto a gas/liquid separator (10).
 10. The process as claimed in claim 9,characterized in that the reaction medium exiting from the static mixer(3) and introduced into the cyclonic reactor (4) has a degree ofreaction progression of between 80 and 95%.
 11. The process as claimedin claim 9 or 10, characterized in that it is carried out in a device asclaimed in one of claims 1 to
 7. 12. The process as claimed in claim 9or claim 10, characterized in that the liquid medium introduced into thestatic mixer (3) comprises at least two reactants A and B.
 13. Theprocess as claimed in claim 12, characterized in that it is carried outin a device as claimed in claim
 8. 14. The process as claimed in claim12 or claim 13, characterized in that the reactant A is a reducingagent.
 15. The process as claimed in one of claims 12 to 14,characterized in that the reactant B comprises a halogen having a degreeof oxidation of greater than or equal to zero.
 16. The process asclaimed in claim 15, characterized in that the reactant B comprises atleast one compound chosen from the group consisting of chlorinatedcompounds, brominated compounds and permanganates.
 17. The process asclaimed in claim 16, characterized in that the chlorinated compounds arechosen from the group consisting of chlorine, chlorine dioxide, sodiumhypochlorite, calcium hypochlorite, hypochlorous acid and solid forms ofchlorine.
 18. The process as claimed in claim 16, characterized in thatthe brominated compounds are chosen from the group consisting from thegroup consisting of bromine, sodium hypobromite, calcium hypobromite andhypobromous acid.
 19. The process as claimed in claim 12 or claim 13,characterized in that the reactant A is an oxidizing agent.
 20. Theprocess as claimed in claim 19, characterized in that the reactant Bcomprises at least one compound chosen from the group consisting ofsodium chlorite, cyanide compounds, sulfur compounds and ferrous iron.21. The process as claimed in one of claims 12 to 20, characterized inthat the reactant A comprises at least one peroxide chosen from thegroup consisting of hydrogen peroxide and alkali metal peroxides. 22.The process as claimed in claim 21, characterized in that the reactant Acomprises hydrogen peroxide.
 23. The process as claimed in claim 12,characterized in that the reactant B is an aqueous effluent comprisingchlorine and the reactant A comprises an aqueous hydrogen peroxidesolution.
 24. The process as claimed in claim 23, characterized in thatthe aqueous effluent comprises hypochlorite ions and/or chlorine. 25.The process as claimed in claim 24, characterized in that the aqueouseffluent comprises: from 1 mg/l to 10 g/l and preferably from 10 mg/l to4 g/l of Cl₂; and from 1 mg/l to 250 g/l and preferably from 10 mg/l to130 g/l of NaClO.
 26. The process as claimed in one of claims 9 to 25,characterized in that it is carried out under the following conditions:temperature: from 0 to 110° C., preferably from 20 to 80° C.; pressure:0.5 to 3 bar, preferably from 0.9 to 1.3 bar; pH: from 1 to 14,preferably from 5 to 12; residence time in the static mixer: 0.001 to100 seconds, preferably from 0.02 to 10 seconds; residence time in thecyclonic reactor: 10 to 400 seconds, preferably 20 to 100 seconds; andthe gas/liquid separator (10) is a coalescer, the operating velocity ofwhich is from 0.01 to 1 m/s, preferably from 0.05 to 0.8 m/s.